A motor driving apparatus that is used to control driving of a motor includes a converter circuit that converts a commercial AC power supply to a DC power supply, a bus-voltage smoothing capacitor that smooths an output DC voltage of the converter circuit to generate a stable bus voltage, and an inverter circuit that carries out switching of the bus voltage generated by the bus-voltage smoothing capacitor in order to generate a driving AC power supply having an arbitrary frequency and voltage to be supplied to the motor.
The inverter circuit is configured by connecting a predetermined number of semiconductor switching elements such as IGBTs in a series-parallel combination in a bridge configuration between positive and negative bus-bars that are connected to both terminals of the bus-voltage smoothing capacitor. A freewheeling diode is connected to each of the semiconductor switching elements in inverse parallel. Specifically, an inverter circuit for a three-phase motor is described as an example. The inverter circuit has a configuration in which three sets of two semiconductor switching elements connected in series are provided in parallel between a positive bus-bar and a negative bus-bar, and three-phase output terminals are configured by three upper-lower-arm connection ends between three upper-arm semiconductor switching elements and three lower-arm semiconductor switching elements.
Meanwhile, a motor, in which its driving is controlled by this motor driving apparatus, performs a power consuming operation (a power running operation), and a power regenerating operation (a regenerative operation) based on a relation between a rotational speed of the motor and a synchronous speed determined by a frequency of driving power supplied from the inverter circuit. For example, a motor used in a crane facility performs a power running operation at the time of hoisting, and performs a regenerative operation at the time of lowering. In the motor driving apparatus, when regenerative power generated by the motor during its operation is input to an output terminal of the inverter circuit, the bus voltage rises. When the bus voltage rises excessively, a protecting circuit is activated, and the motor operation is stopped.
Therefore, in order that regenerative power generated by a motor can be effectively used as motor driving power (which is referred to as “power assist” in this specification), a configuration has been proposed in, for example, Patent Literature 1, in which a buck-boost chopper circuit is provided in parallel to a bus-voltage smoothing capacitor, and a power-assisting electric storage device is provided, in which its charge and discharge is controlled by this buck-boost chopper circuit.
Specifically, the buck-boost chopper circuit has a configuration, in which a series circuit of two semiconductor switching elements such as IGBTs is connected between positive and negative bus-bars (that is, between both terminals of the bus-voltage smoothing capacitor), where a freewheeling diode is connected to each of the semiconductor switching elements in inverse parallel, and a reactor is provided between a connection end between these two semiconductor switching elements and a positive terminal of the power-assisting electric storage device. A negative terminal of the power-assisting electric storage device is connected to the negative bus-bar.
In this buck-boost chopper circuit, a step-down operation is performed by the semiconductor switching element on the positive bus-bar and the reactor, and a boost operation is performed by the semiconductor switching element on the negative bus-bar and the reactor.
This configuration is described using the example in which a motor is used in a crane facility. When regenerative power, generated by the motor at the time of lowering the crane, flows into the inverter circuit, the semiconductor switching element on the positive bus-bar in the buck-boost chopper circuit is first turned on to operate as a buck chopper circuit in order to supply and store, power by an increase in the bus voltage due to the regenerative power, to and in the power-assisting electric storage device. In this state, when the semiconductor switching element on the positive bus-bar is turned off, and simultaneously, the semiconductor switching element on the negative bus-bar, which is a boost chopper circuit element, is turned on, then a discharge current from the power-assisting electric storage device flows through the reactor to the semiconductor switching element on the negative bus-bar. Energy biasing the current to flow in one direction is stored in the reactor.
Therefore, when the semiconductor switching element on the negative bus-bar, through which the discharge current is flowing, is turned off, the current-urging energy stored in the reactor causes the discharge current from the power-assisting electric storage device to be supplied to the positive terminal of the bus-voltage smoothing capacitor through the freewheeling diode connected in inverse parallel to the turned-off semiconductor switching element on the positive bus-bar. Accordingly, the bus-voltage smoothing capacitor is charged. The voltage of this bus-voltage smoothing capacitor is supplied to the inverter circuit, and is then used for motor driving power at the time of a power running operation of the motor.
According to the technique proposed in Patent Literature 1 as described above, regenerative power generated by the motor can be effectively used as motor driving power. Therefore, it is possible to obtain a more energy-efficient motor driving apparatus.
There has been a known motor driving apparatus with a configuration in which, in order to individually drive a plurality of motors incorporated in a numerical-control machine tool or an industrial machine, for example, a plurality of inverter circuits are provided in parallel to an output terminal of a single converter circuit. This motor driving apparatus is referred to as “multiaxial driving apparatus” in this specification for convenience of explanation.
When the technique proposed in Patent Literature 1 is applied to the multiaxial driving apparatus as described above, it is necessary to provide a power-assisting electric storage device and a buck-boost chopper circuit. This results in a problem that the size of the entire apparatus increases, and the number of maintenance parts increases.
In Patent Literature 2, for example, a technique has been disclosed to this problem, in which in order to downsize the entire apparatus, and reduce the number of maintenance parts, a semiconductor switching element in an inverter circuit is also used as a semiconductor switching element in a buck-boost chopper circuit.
That is, in Patent Literature 2, an inverter control circuit and a charge-discharge chopper control circuit are provided. The inverter control circuit controls switching elements in the inverter circuit to cause the inverter circuit to perform an inverter operation to drive the motor. The charge-discharge chopper control circuit controls the switching elements in the inverter circuit to cause the inverter circuit to charge and discharge the power-assisting electric storage device. Outputs of both the control circuits are connected to a control terminal of the inverter circuit through a selector switch. The inverter circuit connects at its output terminal to the motor and to one end of a reactor through selector switches. A positive terminal of the power-assisting electric storage device is connected to the other end of the reactor.
In the technique disclosed in Patent Literature 2, due to this configuration, each of the selector switches is given a control switching signal to switch between a function of the inverter control circuit that is to cause the semiconductor switching elements within the inverter circuit to drive the motor, and a function of the charge-discharge chopper control circuit that is to cause the semiconductor switching elements within the inverter circuit to charge and discharge the power-assisting electric storage device.
For another example, in Patent Literature 3, a technique has been disclosed in which a propulsion driving apparatus is used to charge a power-assisting electric storage device.
According to the techniques disclosed in Patent Literatures 2 and 3, the driving apparatus can cause the inverter circuit to perform an inverter operation and a charge-discharge operation. Therefore, the shared use as the semiconductor switching element in the inverter circuit and as the semiconductor switching element in the buck-boost chopper circuit can be achieved.